Add code coverage check to PR build

[sbethur]

We currently use coverage tool to report the code coverage from unit and system tests. See
e.g:

nimi-python/generated/nidcpower/tox-system_tests.ini

Line 32 in 266e87f

nidcpower-coverage: coverage report --rcfile=../../tools/coverage_system_tests.rc

nimi-python/tox-travis.ini

Line 46 in 266e87f

build_test: coverage report

Logs:

• https://app.travis-ci.com/github/ni/nimi-python/jobs/576582446#L2052
• https://github.com/ni/nimi-python/runs/7326743697?check_suite_focus=true#step:3:1027

We should looking into setting a threshold and have the PR build fail if the coverage is below it.

[ni-jfitzger]

If we do set a threshold, we may need to combine the coverage for the unit tests and session tests for some files, first, as @bkeryan suggested in #2015 (comment) (perform the check as a final step after all of our other checks).